According to Kevin Gill, like this. Created using elevation data from NASA's Mars Reconnaissance Orbiter, this rendering shows what Mars would have looked like if water was present on the surface. It reveals a massive ocean on one side of the planet, and one of the longest valleys in the solar system, Vallis Marineris. Elsewhere, over on the left you can see the peaks of Olympus Mons, Pavonis Mons, Ascraeus Mons and Arsia Mons poking up out of the atmosphere. Gill explains how he made the image:

This is a view of the Western hemisphere with Olympus Mons on the horizon beyond the Tharsis Montes volcanoes and the Valles Marineris canyons near the center. The height of the clouds and atmosphere are largely arbitrary and set for the sake of appearance and coverage over the exaggerated terrain elevations (~10 times elevation exaggeration). The eye is about 10,000 km (~6,200 miles) from the surface.

A two dimensional digital elevation model was first rendered in jDem846 (an open-source learning project of mine) using the MRO MOLA 128 pix/deg elevation dataset. In that model, I picked a sea level and scripted it such that terrain at or below that level was flat and blue.

The resulting model was then brought into GIMP were I painted in land features using a NASA Blue Marble Next Generation image for the source textures... I tried to envision how the land would appear given certain features or the effects of likely atmospheric climate.

Obviously, there's a fair deal of artistic license involved in creating a render of what Mars would look like as a planet supporting life. But that doesn't stop the image being a fascinating insight into the possible past life of Mars—and a refreshing change from seeing it as a dusty red ball. [Kevin Gill via Discovery]